Paper Sludge SSCF at 30-C
SSCF of paper sludge at 308C was performed to test the effect
of temperature on cell viability and sugar utilization. Paper
sludge SSCF using S. cerevisiae RWB222 at 308C had a
similar fermentation profile as at 378C, except that the viable
cell numbers remained relatively constant after reaching
36 g/L ethanol, as seen from Figure 2A. Z. mobilis 8b maintained
good cell viability at 308C, and while some cellobiose
and glucose accumulated during the first 50 h, no residual
cellobiose and glucose were detected at the end of fermentation
(Fig. 2B). Residual glucan after 120 h is around
10 g/L and residual xylan is around 2.6 g/L in the solid
residue of SSCF by both Z. mobilis and S. cerevisiae at 308C, a
concentration higher than by S. cerevisiae RWB222 at 378C
as shown in Table I.
Low Concentration Paper Sludge SSCF
A set of experiments was designed to assess the contribution
of ethanol inhibition to the loss of cell viability observed in
paper sludge SSCF at 378C. If ethanol inhibition were a
major factor responsible for loss of viability, then good cell
viability would be expected in SSCF experiments at lower
initial cellulose concentrations. As seen from Figure 3A and
C, S. cerevisiae RWB222 established high viability at both 30
and 378C under conditions where the final ethanol reached
15 g/L, suggesting that ethanol concentration is the major
factor for declining viability at high substrate concentrations
for SSCF with this organism. However, even at a reduced
final ethanol concentration, cell viability in SSCF with Z.
mobilis 8b still decreases dramatically at the end of fermentation
at 378C (Fig. 3B). For high concentration paper
sludge SSCF with Z. mobilis, good cell viability is again
observed at 308C (Fig. 3D). These results indicated that one
or more factors other than ethanol inhibition contributed
substantially to loss viability in SSCF using Z. mobilis 8b at
378C.
Avicel SSF
SSF was carried out with Avicel, a model substrate that is
free of inhibitors and far less viscous than paper sludge at
the start of a high solids fermentation. Figure 4 shows the
fermentation profile of Avicel SSF at 378C by S. cerevisiae
RWB222 and Z. mobilis 8b, in which viable cell number